1. Field of the Invention
The present invention is a technology that relates to an exposure apparatus used in the transfer process of the lithographic process for manufacturing highly integrated semiconductor circuit devices.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used in this photolithographic process has a mask stage that supports the mask, and a substrate stage that supports the substrate, and transfers the pattern of the mask onto the substrate through a projection optical system while successively moving the mask stage and the substrate stage.
There has been demanded in recent years for higher resolution projection optical systems in order to handle the much higher levels of integration of device patterns. The shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system, the higher the resolution of the projection optical system. Consequently, the exposure wavelength used in exposure apparatuses has shortened year by year, and the numerical aperture of projection optical systems has increased. Furthermore, the mainstream exposure wavelength is currently the 248 nm assigned to KrF excimer laser, but an even shorter wavelength 193 nm assigned to ArF excimer laser is also being commercialized. In addition, as with resolution, the depth of focus (DOF) is important when performing exposure. The following equations express the resolution R and the depth of focus δ, respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficients. Equations (1) and (2) teach that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ decreases.
If the depth of focus δ becomes excessively narrow, then it will become difficult to make the surface of the substrate coincide with the image plane of the projection optical system, and there will be a risk of insufficient margin during the exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, Reference Document 1 (PCT International Publication WO99/49504), as a method to substantially shorten the exposure wavelength and increase the depth of focus. In this liquid immersion method, a gap between the lower surface of the projection optical system and the surface of the substrate is filled with a liquid, such as water or an organic solvent, thus taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally about 1.2 to 1.6), thereby improving the resolution as well as increasing the depth of focus by approximately n times.
As far as is permitted, the disclosure of the above PCT International Publication is hereby incorporated by reference.
Incidentally, when the projection optical system and the substrate move relatively to one another, the liquid in the immersion area between the projection optical system and the substrate surface begins to move while being dragged in the direction of the movement of the substrate. In particular, a phenomenon occurs wherein, if the relative movement occurs at high speed in order to improve throughput, then the liquid separates from the lower surface of the projection optical system. Consequently, the separation of the liquid from the lower surface of the projection optical system is prevented by increasing the flow rate of the liquid supplied to the immersion area
Nevertheless, if the flow rate of the liquid is increased, then there is a problem in that the generation of vibrations and bubbles in accordance with the movement of the liquid hinders the formation of the circuit pattern.